GB2131167A

GB2131167A - Alignment of mask and semiconductor wafer in exposure apparatus

Info

Publication number: GB2131167A
Application number: GB08331280A
Authority: GB
Inventors: Kazuo Takahashi; Hiroshi Sato; Masao Kosugi
Original assignee: Canon Inc
Current assignee: Canon Inc
Priority date: 1982-11-26
Filing date: 1983-11-23
Publication date: 1984-06-13
Also published as: JPH0114697B2; FR2536873A1; GB2131167B; FR2536873B1; JPS5998525A; US4617469A; GB8331280D0; DE3342719C2; DE3342719A1

Description

1 GB 2 131 167 A 1

SPECIFICATION

An exposure apparatus 5Background of the invention

Field of the invention

The present invention relates to an exposure apparatus, and more particularly, a die-by-die, or shot by shot exposure apparatus for manufacturing semiconductor circuits, and provide an apparatus which can reduce time required in the operational step from the alignment to the exposure at each shot.

Description of the prior art

There is generally known semiconductor manufac- 80 turing exposure systems in which the pattern on a mask or reticle (hereinafter called generally---mask") as an original is printed on a wafer. Recently, more attention has been attracted to an die-by-die type exposure system (so-called stepper).

In such a stepper, the pattern on a mask is projected and printed on a wafer at a reduced scale through a projection optical system having its magnification less than unit. To avoid any overlap ping in the exposed areas at each shot, the mask and wafer are moved relative to each other so that the pattern will be printed on all the effective areas of the wafer through a plurality of shots.

With the die-by-die exposure utilizing the above projection at the reduced scale, the pattern on the mask can be made larger than the pattern to be printed on the wafer by a factor of the inverse number of the magnification in an imaging system.

Thus, difficulties in manufacturing marks can be eased so that finer patterns can be printed on wafers. 100 The stepper is generally classified into two types, that is, (1) OFF-AXIS alignment type and (2) TTL (Through The Lens) ON-AXIS alignment type.

In the off-axis alignment system, the pattern on a wafer is aligned with an alignment optical system located out of a projection optical sytem without passing through the latter. Thereafter, the wafer is accurately moved below the projection optical sys tem such that the wafer pattern will be aligned indirectly with the pattern on a photo-mask.

In the TTL on-axis alignment system, the patterns on a photo-mask and wafer are simultaneously observed to align them directly with each other. The stepper should have its precision of alignment in the order of 0.1 to 0.2 Lm, The off-axis alignment system 115 have larger number of factors of error, because of the fact that the wafer pattern is aligned indirectly with the photo-mask pattern and also cannot deal with in-plane-distortion in the wafer. To improve the precision of alignment, therefore, it is highly desir able to develop the TTL on-axis alignment system.

The prior art TTL on-axis alignment type steppers have the following alignment process:

(1) Displacement, i.e., the misalignment between a mask and wafer is measured through a binocular microscope which is positioned in an optical path of illumination. For example, as disclosed in U.S.

Patent 4,167,677, alignment marks of the particular shape on the mask and wafer are scanned by a laser beam such that the degree of misalignment can 130 automatically be measured in an photoelectric manner.

(2) The degree of misalignment measured in the above step (1) is connected by moving a stage on which the mask or wafer is placed.

(3) Whether or not the correction is properly made is verified by the use of the binocular microscope which is located in the optical path of illumination.

(4) The binocular microscope is retracted out of the optical path.

However, the TTL on-axis alignment system has a problem in that longer time is required in the operation from the beginning of the alignment to the exposure at each shot.

Further, atthe entire exposure step, two alignment marks to be observed through the binocular microscope at each shot should be maintained at such a state that the alignment marks can be used. In addition, if the alignment mark on the outer margin of the exposed region on the wafer is partially broken, no alignment operation can be made. Summary of the invention

It is a principal object of the present invention to provide an exposure apparatus which can be operated at higher speed.

Another object of the present invention is to provide an apparatus for detecting misalignment between an original and a radiation sensitive member when they are overlapped one above another, which apparatus requires a reduced time from the beginning of the detection of misalignment to the end of the exposure.

Still another object of the present invention is to provide an apparatus wherein the time required for a binocular microscope for detecting the radiation sensitive member through the original, is retracted out of the optical path after it has been verified by the binocular microscope that the misalignment was corrected.

Afurther object of the present invention is to provide an alignment-scope which can be operated for alignment in binocular or ocellar mode.

In the preferred embodiment of the present inven- tion which is constructed to accomplish the above objects, one of two binocular microscopes for alignment is positioned in an optical path of illumination and the other is located out of the optical path. The one microscope is used to detect a misalignment between the mask and wafer. Based on the detected misalignment, the mask or wafer are moved relative to each other to correct the misalignment while at the same time this microscope is retracted out of the optical path. At this time, the change in the absolute position of the mask is verified by the other microscope.

These and other objects, features and advantages of the present invention will become more apparent upon a consideration of the following description of the preferred embodiment of the present invention taken in conjunction with the accompanying drawings. Brief description of the drawings

Figure 1 is a view of one embodiment according to the present invention; 2 GB 2 131 167 A 2 Figure 2 is a view showing the field of view in a microscope in an optical path of exposure;

Figure 3 is a view showing the field of view in another microscope out of the optical path; and

Figure 4 is a view showing a modification with respect to the embodiment of the present invention shown in Figure 1.

Description of the preferred embodiment

Referring to Figure 1, the apparatus according to the present invention comprises two microscopes (alignment-scopes) each including an objective lens 1 or Vand an reflection mirror or prism 2 or 2'. Each of the alignment-scopes further includes main bodies AS or AS'which contains a source of light, a optical scanning device, a light receiving device, a viewer and others. The alignment-scopes will not be further described for the sake of simplicity of explanation since they are described in detail in the above U.S. Patent 4,167,677.

The apparatus also comprises a reduction- 85 projection optical system 3 which serves to place a wafer 4 which has a radiation sensitive (such as photosensitive) surface and a mask 5 in optical conjugation with each other. An illumination system IA is located above the mask 5 to illuminate the region of a circuit pattern thereon. The frame of the apparatus includes mask reference marks Wand 6 (the latter is not shown) used to detect the absolute position of the mask and alignment marks 7 and 7' on the mask which are used to detect the position of the mask relative to the mask reference marks 6 and W. The marks 6, 7 and W, 7 are respectively coarsely aligned by the use of the respective alignment scopes AS and AW. This will be referred to "the preset of the masC. The mask 5 further includes alignment marks 8 and 8' near the circuit pattern thereof which are used to align the mask and wafer with each other. The wafer 4 has alignment marks 9 and Wthereon. Although Figure 1 shows only a pair of alignment marks 9 and 9', the actual number of such alignment marks is equal to that of shots performed in this apparatus.

The apparatus further comprises a Y-stage 10 for moving the wafer in the Y-direction, a high-precision scale 11 mounted on the Y-stage 10, and X-stage 12 for moving the wafer in the X-direction, a high precision scale 13 mounted on the X-stage and scale readers R and Wfixed to the frame of the apparatus (not shown). The stages 10 and 12 are used to step the wafer 4. The apparatus further comprises a mask stage 14 for moving the mask 5 with fine motions in the X- and Y-directions. The mask stage 14 includes an opening (not shown) through which the image of the mask 5 passes. The mask stage 14 is driven by a drive D in the X- and Y-directions until a misalign ment detected by the alignment-scope AS becomes equal to a measurement in the alignment-scope AS'.

The mask alignment mark 8 and wafer alignment mark 9', which are to be observed through the objective lens 1 in the alignment-scope, are located in the illumination optical path of exposure. On the other hand, the mask reference mark Wand mask alignment mark 7', which are to be observed through the objective lens Vin the alignment-scope, are located out of the above optical path.

The fields of view observed through the objective lenses 1 and 1' are shown in Figures 2 and 3, respectively.

Referring to Figure 2, the mask alignment mark 8 consists of two sets of parallel lines 8a, 8b and 8c, 8d, these sets being inclined oppositely relative to each other. The wafer alignment mark 9'consists of two lines 9a and 9b each of which is observed to position between the parallel lines of the corresponding set in the mask alignment mark 8.

Referring to Figure 3, the mask alignment mark 7' consists of two sets of parallel lines 7'a, 7'b and 7'c, 7'd while the mask reference mark Wconsists of two lines Wa and Wb. These lines are arranged in the same manner as those of Figure 2.

The lines of the marks are scanned respectively by scanning laser beams L1 and L2 to photoelectrically detect the degree of misalignment in the following manner:

If there is a relative misalignment between the mask and wafer, the spacing between the mark lines 8a and 9a becomes non-equal to that between the mark lines 9a and 8b. Simultaneously, the spacing between the mark lines 8c and 9b becomes non- equal to that between the mark lines 9b and 8d. This is utilized to detect the degrees of misalignment between the mask and wafer both in the X- and Y-directions. If the relative misalignment A between the mask 5 and the wafer 4 is detected through the objective lens 1 in the alignment-scope AS, the mask stage 14 is moved with fine motion in the X- and Y-directions to correctthis misalignment A. At this time, any change in the relative position between the mask 5 and the wafer 4 is verified through the objective lens 1'of the alignment-scope AS'located out of the illumination optical path. If the mask stage 14 is moved by a distance -A to correct the misalignment A between the mask 5 and the wafer 4, it can be verified depending on whether or not the spacings between the mark lines 7'a, Wa; Wa, 7b; 7'c 6% and 61, 7'd become coincide with the distance -A, as shown in Figure 3.

At the same time as the mask stage 14 is moved, the reflection prism 2 or the objective lens 1 and reflection prism 2 in the alignment-scope AS are retracted out of the illumination optical path. Thus, when the correction of the relative misalignment between the mask and wafer is verified through the objective lens Vin the alignment-scope AS', the -- reflection prism 2 has already be retracted out of the illumination optical path. The procedure can immediately be gone to the exposure step without any stand-by time as in the prior art. Thus, time required in the operation from the alignment to the exposure at each shot can be reduced to improve the throughput.

Although the objective lens Vin the alignmentscope AS' may be stationary, it is preferred that it is movable since this alignment-scope AS'may be used as a microscope located within the illumination optical path as will be described below.

Namely, although Figure 1 shows the left-hand microscope located within the illumination optical path and the right-hand microscope located out of the optical path, it is desirable that the right-hand A 1 3 GB 2 131 167 A 3 microscope AS'is movable in the X- and Ydirections such that it can be located within the illumination optical path. Because of the movability, it is possible that the alignment for the first shot is performed with two microscopes AS and AS' both inserted in the illumination optical path, and then the alignments forthe subsequent shots are performed in the manner of the present invention as described above, i.e., with the microscope 1' at the position shown in Figure 1.

In the preferred embodiment of the present invention, the mask reference marks 6 and 6' are positioned close to the mask 5. However, the mark reference marks 6 and 6'may be provided integrally with the projection optical system 3, for example, on the mirror barrel of the projection optical system 3 at the top thereof to provide no relative offset between the mask reference marks and the projection optical system 3. 20 The positions of the mask reference marks 6 and 6' 85 and the positions of the mask alignment marks 7 and 7' relative to the mask reference marks are not limited to those shown in Figure 1. The configuration of the alignment marks is not limited to that shown in the drawings as far as they 90 can detect any misalignment in the X-direction or the Y-direction.

Although the illustrated embodiment has been described in which the misalignment between the mask and wafer is measured through one of the objective lenses, and the mask stage is moved for alignment while monitoring, with the other objective lens, the positional relationship between the mask alignment mark and mask reference mark, the wafer stage instead of the mask stage may be moved with fine motion.

If the wafer stages has high-precision scales, it can be considered that after the relative misalignment between the mask and wafer has been measured through the microscope (ocellar or binocular micro scope) within the illumination optical path, the wafer stages 10 and 12 are moved to correct the relative misalignment between the mask and wafer, relying upon the high-precision scales, by the amount read by the reader R and R'without any verification of the corrected positional misalignment through the mic roscopes. At the same time, the microscope is retracted out of the illumination optical path for exposure. If the relative misalignment is measured by the binocular microscope, the rotational misalign ment between the mask and wafer can be corrected.

Each of the wafer stages may have a high precision laser interferometer in place of the high precision scale. If the wafer stages having high precision scales are incorporated into the apparatus, the correction of alignment may be effected one through several shots, in place of that the alignment is carried out based on the TTL method at each shot, and, at the remaining shots, the wafer stage is stepped through a predetermined distance without alignment operation, relying on the precision of the scale, and then the wafer is exposed.

In addition, the binocular microscope may freely be positioned in the apparatus and particularly it may be retracted to any suitable position out of the illumination optical path. As shown in Figure 4, therefore, the relative misalignment between the mask and wafer at a certain shot may be measured at a shot immediately before said one shot. In this case, at the same time as the termination of the exposure, the wafer may be stepped in consideration with the amount of correction thus determined beforehand.

It is apparent that if two microscopes are made movable, the alignment may be carried out within the illumination optical path in the binocular manner in the manner of the prior art. In the other case, the alignment may be effected within the illumination optical path in the ocellar manner in accordance with the present invention.

In summary:

(1) Displacement between the mask and wafer is measured through the ocellar alignment microscope located within the illumination optical path. At the same time, the absolute position of the mask is detected through the other ocellar microscope located out of the exposure optical path.

(2) The mask stage is moved depending on the detected displacement and at the same time the correciton of this displacement is confirmed or verified through the ocellar alignment microscope located out of the exposure optical path. Simultaneously, the alignment microscope within the exposure optical path is retracted out of the expo- sure optical path. Thereafter, the exposure step is performed. Thus, the operational time from the alignment to the exposure can effectively be reduced.

The prior art utilized two sets of alignment marks at minimum at each shot. However, if only one set of alignment marks are effectively present on the wafer for any reason, no alignment can be made at that shot. In accordance with the present invention, the alignment can be made if only one set of alignment marks at minimum are present. Even though only one set of alignment marks on both the mask and wafer extend over the outer margin of the wafer, the alignment can be made according to the present invention. This is particularly effective if a plurality of chips are printed at a single shot.

As be understood from the foregoing, the apparatus according to the present invention can perform its inherent function in which the advantage of the TTL alignment system is utilized to execute the alignment at each shot with binocular detection with higher precision, and in addition, can improve the productivity while maintaining the high- precision alignment, by effecting the TTL alignment at each shot with ocellar detection and combining the subsequent operations.

While the invention has been described with reference to the structures disclosed herein, it is not confined to the details set forth and this application is intended to cover such modifications or changes as may come within the purposes of the improvements or the scope of the following claims.

Claims

1. An exposure apparatus comprising usable 4 GB 2 131 167 A 4 with an original and a radiation sensitive member for receiving the image of said original, comprising:

a first detection means including a sensor retractably placed in the optical path passing through said original, said sensor being adapted to sense said original and said radiation sensitive member to detect a relative displacement between said original and said radiation sensitive member when said sensor is set within said optical path; a second detection means, including a sensor located out of the optical path, for detecting the position of one of said original and radiation sensitive member relative to an absolute coordinates in said apparatus; and means for producing relative movement between said original and radiation sensitive member in response to the output of said second detection means to compensate said relative displacement.

2. An exposure apparatus as defined in claim 1, further including means for stepping said radiation sensitive member.

3. An exposure apparatus as dQfined in claim 1, further including means for stepping said radiation sensitive member and a projection optical system for projecting the image of said original onto said radiation sensitive member.

4. An exposure apparatus as defined in claim 3 wherein said projection optical system is adapted to project the image of said original onto said radiation sensitive member at a reduced scale.

5. An exposure apparatus as defined in claim 1 wherein each of said original and radiation sensitive member includes at least one alignment mark formed thereon to be sensed by said first detection means.

6. An exposure apparatus as defined in claim 1 wherein said second detection means is adapted to measure the displacement of a setting mark on said original relative to a reference mark in said appar atus.

7. An exposure apparatus as defined in claim 1 wherein said moving means includes a movable stage for holding said radiation sensitive member and wherein said second detection means is adapted to measure the position of said stage relative to said 110 apparatus.

8. An exposure apparatus as defined in claim 7 wherein said second detection means includes a scale.

9. An exposure apparatus as defined in claim 7 115 wherein said second detection means includes a laser interferometer.

10. An method comprising the steps of:

detecting, by an objective lens section, a relative displacement between an original and a radiation 120 sensitive member through said original; retracting the objective section out of the optical path passing through said original after said relative displacement has been detected; detecting the position of at least one of said 125 original and radiation sensitive member relative to the absolute coordinates; moving said original and radiation sensitive mem ber relative to each other to align them until the detected value compensate with said relative displacement; and exposing said radiation sensitive member to an image of said original.

11. A method as defined in claim 10, further including the step of producing stepping relative movement between said original and photosensitive member after said radiation sensitive member has been exposed to the image of said original.

12. A method as defined in claim 10 wherein the image of said original is projected at a reduced scale.

13. A method as defined in claim 10 wherein said step for detecting the position includes the step of measuring the displacement of a mark on said original relative to the absolute reference.

14. A method as defined in claim 10 wherein said step of detecting the position includes the step of measuring the displacement of said radiation sensitive member relative to the absolute reference.

15. An apparatus usable with a mask having a first alignment mark and a mask alignment mark and a wafer having a second alignment mark matching said first alignment mark, comprising:

a mask stage for supporting and moving said mask; a wafer stage supporting and moving a wafer; a projection optical system for projecting an image of said mask onto said wafer at a reduced scale; illumination means for defining an illumination optical path for said mask; means for providing a reference mark matching said mask alignment mark and which is used as absolute coordinates; detection means, including sensor means retractable out of said illumination optical path, for sensing said first and second alignment marks to detect a relative displacement between said mask and wafer; measure means, including sensor means located out of said illumination optical path, for sensing said mask alignment mark and said reference markto measure the position of said mask relative to the absolute coordinates; and drive means for moving said mask stage until the measured value in said measure means compensate with the relative displacement.

16. An apparatus as defined in claim 15 wherein said drive means is adapted to move said mask stage along rectangular coordinates.

17. An apparatus as defined in claim 15 wherein said mask includes a third alignment mark cooperating with said first alignment mark and said wafer includes a fourth alignment mark matching said third alignment mark and wherein the sensor means of said measure means is adapted to sense said third and fourth alignment marks and to cooperate with said detection means to detect the relative displacement between said mask and wafer.

18. An apparatus comprising:

a mask stage for supporting a mask; a wafer stage for supporting and moving a wafer; illumination means for defining an optical path for illuminating said mask; detection means, including sensor means retractable out of said optical path, for sensing said wafer through said mask to detect a relative displacement between said mask and wafer; o, 9 GB 2 131 167 A 5 measure means for measuring the position of said wafer stage relative to said apparatus; and drive means for moving said mask stage until the measured value in said measure means compen5 sates with said relative displacement.

19. An apparatus as defined in claim 18 wherein said drive means is adapted to move said wafer stage along rectangular coordinates.

20. An apparatus as defined in claim 18 wherein said sensor means has one field of view.

21. An apparatus as defined in claim 18 wherein said senor means has two independent fields of view.

22. An apparatus as defined in claim 18 wherein said measure means includes a scale and reader means for reading said scale.

23. Apparatus for achieving a desired positional relationship between first and second objects, comprising:

means defining first and second relatively movable frames of reference for said first and second objects respectively; means defining a datum frame of reference; said first and second frames being movable relative to the datum frame; means for detecting the relative position of the first and second frames; and means responsive to said detecting means for subsequently changing the relative position of said first and second frames by reference to said datum frame.

24. A method of achieving a desired positional relationship between first and second objects each movable relative to a datum poition, including detecting the relative position of the first and second objects with detector means responsive to positional information derived from said first and second objects, detecting the position of one of said first and second objects relative to the datum position with detector means responsive to positional information derived from said datum position and said one of the objects, and thereafter moving one of said objects to assume a position related to said datum position which achieves a desired positional relationship between the first and second objects.

25. A method according to claim 24 including rendering inoperative the detector means responsive to the first and second objects after said derivation of the information relating to the relative position of said objects.

26. A method according to claim 25 including processing the second of the objects to form a record thereon with information on the first of said objects, subsequent to the achievement of said desired positional relationship.

27. Exposure apparatus substantially as herein described with reference to the accompanying drawings.

28. A method of aligning a mask and wafer substantially as herein described with reference to the accompanying drawings.

Printed for Her Majesty's Stationery Office, by Croydon Printing Company Limited, Croydon, Surrey, 1984. Published by The Patent Office, 25 Southampton Buildings, London, WC2A 1 AY, from which copies may be obtained.